1. Field of the Invention
The present invention relates to contact lenses for correcting presbyopia, and more particularly to contact lenses for correcting presbyopia that are scaled based upon pupil size as it relates to refractive error to ensure that the design provides the same visual experience independent of the level of ametropia or refractive error. The present invention also relates to a method for adjusting the optical designs for lenses for correcting presbyopia to account for changes in pupil size as it relates to ametropia.
2. Discussion of the Related Art
As individuals age, their eyes are less able to accommodate, or bend their natural or crystalline lens, to focus on objects that are relatively near to the observer. This condition is known as presbyopia. More specifically, when an individual is born, the crystalline lens is pliable which makes it capable of a high degree of accommodation. As the individual ages, the crystalline lens gradually becomes more rigid and thus less able to accommodate. Similarly, for persons who have had their natural or crystalline lens removed and an intraocular lens or IOL inserted as a replacement, the ability to accommodate is absent. Although the intent of an accommodating IOL is to address this potential shortcoming, current accommodating IOL designs and concepts are relatively new and continue to evolve.
Various classes of contact lens and intra-ocular designs have been offered for the treatment of presbyopia. These include bi-focal and multi-focal contact lenses of various forms, including concentric rings, aspheric designs, as well as diffractive designs. These designs are typically depicted in the patent literature by their power profiles. Even if described by surface or other attributes, the power profile for a given design may be determined.
An example of a power profile for a concentric ring type design is illustrated in FIG. 1. The horizontal axis shows the radial position from the center of the lens in millimeters. The vertical axis shows the contact lens power, in diopters (D), relative to the label power of the contact lens. This particular design consists of five concentric rings. The contact lens power plotted here is relative to the label power. The label power is the power required to compensate for the level of ametropia or refractive error of the patient. For example, it may be determined by an eye care professional that an individual with myopia or nearsightedness requires −2.75 D lens to correct their ametropia. The label power of the contact lens selected will be −2.75 D.
For a particular design, such as the one illustrated in FIG. 1, there is required a set of lenses of a range of label powers. Typically a particular design such as the one illustrated in FIG. 1 is provided with label powers from −12.00 D to 8.00 D in 0.25 D increments. The prior-art (patent or otherwise) typically describes an optical design intended for the treatment of presbyopia for a single label power. The method for determining the designs at other label powers is not specified but implied in the description of the design at the nominal power. The implied method for the design from FIG. 1 to create the set of designs to cover a range of label powers is to take the nominal design and add to it a constant power equal to the label power. The set of power profiles for this design in 1.0 D increments from −8.0 D to +6.0 D label powers is illustrated in FIG. 2.
There are many forms of bi-focal or multi-focal contact lenses for the correction of presbyopia. These design forms include concentric rings, aspheric designs, as well as diffractive designs. All of these designs function by providing a range of powers within the pupil of the eye. For example, a concentric ring design may have a central ring that provides powers that are nominally equal to the power required to correct the distance vision of the subject, an adjacent ring that provides near powers, and an outer ring that also provides distance powers. There may also be versions or variations with intermediate powers to address situations between near and far distances, for example, computer screen viewing. An aspheric design may be considered a multi-focal or progressive type design that provide powers for a given pupil size that gradually change from being plus to distance in the center of the lens and providing powers for near vision correction to having distance powers at the edge of the pupil to provide distance vision correction.
Pupil size depends upon a number of factors, including light level. Much of the design work and prior art for presbyopic designs is concerned with optimizing the design performance for a range of light levels and thus pupil sizes. In designing these lenses for presbyopes, the pupil size is taken into account. The approach to doing this depends upon the intent of the design. One goal may be to make a design independent of pupil size so that vision will stay constant as light levels change and pupil sizes change. Alternatively, the intent may be to provide a lens that gives preference to near vision for small pupils and distance vision for large pupils such as is done by many of the center near designs. Or, the intent may be to provide a lens that gives preference to distance vision for small pupils and near vision for large pupils such as is done by many of the center distance designs. The design possibilities and permutations are essentially endless.
Pupil size also depends upon the level of ametropia. Referring now to FIGS. 3, 4 and 5, there is illustrated in graphical format the relationship between pupil size and refractive error for a given luminance level. More specifically, FIGS. 3-5 illustrated pupil size data collected at 2.5, 50, and 250 cd/m2 (candela per square meter) luminance levels. This data is for subjects greater than forty (40) years old representing the presbyopic population. The pupil size data on the vertical axis is plotted against the refractive error in Diopters on the horizontal axis. As may be seen from the figures, the pupil sizes at all light levels are smaller for hyperopes than for myopes. Accordingly, since the pupil size at a given light level varies with the refractive error, then lenses for the treatment of presbyopia are needed that have their designs scaled based upon pupil size to ensure that the designs perform consistently independent of the refractive error being corrected.